This invention relates to linear oscillating electric motor/alternators, and more particularly to such a machine having permanent magnet excitation and end flux leakage control.
Many power applications produce or require linear reciprocating motion and an energy converter which will convert the linear mechanical motion into electrical energy or will convert the electrical energy into linear reciprocating mechanical motion. Such an energy converter, commonly called a linear oscillating motor/alternator, typically has one member directly connected to the linear reciprocating mechanical member, and the other member is stationary.
The principle of operation of linear reciprocating motion converters can be divided into two main categories: flux switching and flux reversing. In the flux switching type the DC excitation coil which is the source of the main flux, is on the stationary member as are the AC coils. The flux linkage of the AC coils is switched from high to low level by movement of the plunger between its two extreme positions.
In the flux reversing type, the source of electromotive force is carried by the moving member or plunger. The coils are on the fixed member or stator, and the flux linking the coils goes from positive maximum to negative maximum as the plunger moves between its extreme positions. In both flux switching and flux reversing type machines, the change in the flux seen by the Ac coils causes a voltage to be induced in the coils of the same frequency as the oscillation of the plunger. When used as a motor, the stator coils carry the current which generates a driving flux that coacts with the plunger magnet flux to convert electrical to mechanical energy. When used in the alternator mode, the induced voltage delivers current and power to the outside load. The source of the exciting flux may be a coil or it may be a permanent magnet. Use of a permanent magnet for flux reversal machines avoids sliding or flexible current collectors.
Variants of these motors can be classified from yet another view point, depending on whether or not the moving member is used as a flux carrier. In either the flux switching or the flux reversing type, only a part of the magnetic circuit needs to move relative to the other part. When this can be achieved, it results in an extremely light plunger which is usually desirable. However, in such configuration the flux has to cross twice as many air gaps as required in the configurations where in the flux carrier also travels with a moving part of the magnetic circuit.
The flux reversing type permanent magnet motor/alternator has the inherent advantage that the flux linkage around the AC coils varies from positive maximum to negative maximum and therefore, offers the greatest potential for most efficient utilization of the flux generating or force generating structure. Its primary disadvantage, in prior art machines, is the large air gap in the return path of the flux in the magnetic circuit which greatly increases the reluctance of the circuit and decreases the intensity of the flux, and therefore, the power of the machine. Another serious problem in the prior art machines, is the short-circuiting of the flux by the method of attachment of the magnets to the moving body. The magnets are often embedded in the body which offers a short-circuiting path for the flux, thereby decreasing the proportion of the flux linking the AC coils. A third, and potentially the most serious problem, is the flux leakage at the ends of the machine. Unless the machine is designed to confine the flux to a low reluctance magnetic circuit around the coils, the flux at the ends of the machine will be lost in leakage paths, reuslting in large inefficiencies. In addition, hysteresis and eddy current losses in the motor materials at the ends of the machine, as a result of flux changes in the moving body, cause additional large power losses and result in lower efficiencies.
There are many applications for a linear reciprocating permanent magnet electric motor/alternator having a light weight plunger and a stationary stator having no moving contacts and using or producing single phase AC power. This device should have small side pull on the plunger to reduce the loads on the plunger bearings, and the machine must operate with high efficiency yet have a lower manufacturing cost.